This invention relates to an apparatus and a process for preventing stalagmite formation in the paper coating operation.
In the manufacture of coated paper, to obtain paper having a smoother and microporous surface, one or more coatings serving to fill in the roughnesses in the paper surface and thereby to equalize that surface are applied to the uncoated paper. Suitably, an excess of wet coating composition is applied to the paper web by means of a premetering device.
In order to provide a uniform thickness of the coating on the paper web, a doctor, the so-called doctor blade, extending across the width of the paper web is arranged at some distance from the premetering device.
It has been known for many years that at high speeds of a paper web, which high speeds are desired to realize the highest possible production, problems arise in the area downstream of the doctor blade, seen in the direction of travel of the paper web, (the so-called xe2x80x9cdryxe2x80x9d side of the doctor blade) owing to the fact that in that area, across the length of the doctor blade, a liquid or hard edge is formed consisting of coating composition which may or may not be mixed with fibres. In the technical literature the formation of such an edge is called xe2x80x9cstalagmite formationxe2x80x9d or also xe2x80x9cbleedingxe2x80x9d, xe2x80x9cwhiskeringxe2x80x9d or xe2x80x9cfeatheringxe2x80x9d. In the present text the term stalagmite formation will be used.
It is known from the literature that the stalagmite formation assumes more serious proportions as the speed of the paper web increases. At the moment speeds of 1,000 to 1,500 m/min are already customary and for the future even higher speeds are not impossible. Moreover, the gravity of stalagmite formation depends on the composition of the coating, for which every paper maker has his own specific recipe, which coating, however, will always contain a pigment constituent, such as clay or chalk, and a binder, such as a latex or a modified starch. It is desirable both for the quality of the paper and for the production costs thereof to apply a coating having a high solids content. Such a high solids content, however, is also found to promote a strong stalagmite formation at the doctor blade.
The formation of a stalagmite on the xe2x80x9cdryxe2x80x9d side of the doctor blade is highly undesirable, because pieces of coating composition may get loose from this stalagmite, especially when it assumes too large sizes, which pieces, when arriving at the paper web, may damage the paper surface, as they cause streaks and cavities. Furthermore, hard pieces of stalagmite may damage the surface of the rolls over which, in the process of papermaking, the paper web passes after application of the coating. In particular the surface of the so-called calender rolls to be used for the further smoothing of the paper surface is susceptible to this. Replacement of such rolls is very expensive. Finally, in the subsequent process of printing paper hard pieces of coating composition which are still present in the paper after the manufacturing process may lead to problems or to disfigured printed matter. In addition, the occurrence of stalagmite restricts the selection of the raw materials to be used in the coating composition, which may adversely affect the quality of the paper and/or the costs of raw materials.
For a more detailed description of the problem described above, reference can be made to three articles in the xe2x80x9cWochenblatt fur Papierfabrikationxe2x80x9d, namely xe2x80x9cStreichfarben mit hohem Feststoffgehaltxe2x80x94Rheologie und Verarbeitbarkeit beim Bladestreichenxe2x80x9d by G. Engstrxc3x6m in Vol. 6, 1984, pages 184-187; xe2x80x9cFabrikationsstxc3x6rungen durch stalagmitenfxc3x6rmigen Streichfarbenaufbau an den Schabern schnell laufender Streichmaschinenxe2x80x9d by H. P. Hofmann and A. von Raven in Vol. 8, 1986, pages 261-265; and xe2x80x9cBetrachtungenxc3xcber Stalagmitenbildung und Bladexc3xcberkochenxe2x80x9d by D. Eklund and S. Fors in Vol. 10, 1988, pages 400-404. Two articles published in xe2x80x9cTappixe2x80x9d in 1973 show that the disadvantageous effects of stalagmite formation have been recognized for may years. These articles are: xe2x80x9cEffect of coating color rheology on the blade coating processxe2x80x9d by J. P. Modrak in Vol. 56, No. 10, October 1973, pages 70-73 and xe2x80x9cEffect of hydrocoloids on coating color operability and coating paper propertiesxe2x80x9d by E. J. Barbar in Vol. 56, No. 10, January 1973, pages 52-55.
It will be clear from the above that in the paper coating operation it is highly important to prevent stalagmite formation without concessions being required with regard to the desired high speed of the paper web and/or the makeup of the coating composition and/or the properties of the paper to be coated and more in particular with concessions being required with regard to the desired relatively high solids content of the coating composition.
It is therefore an object of the invention to provide an apparatus and a process for fully inhibiting stalagmite formation or at least reducing stalagmite formation to an absolutely harmless level, independently of the speed of the paper web moving along the doctor, independently of the makeup and the solids content of the coating composition and independently of the quality of the paper to be coated.
The invention suitably provides a process for coating paper, which comprises providing a paper web with a layer of coating composition and passing the paper web along a doctor, characterized in that harmful stalagmite formation on the, seen in the direction of movement of the paper web, downstream side of the doctor is prevented by supplying a fluid inhibiting stalagmite formation to an area which, on the one hand, is bounded by the paper web and, on the other hand, by the above side of the doctor.
The fluid inhibiting stalagmite formation is preferably steam, but also applicable is water or a mixture of steam and/or water and/or air and any other fluid that inhibits stalagmite formation and does not adversely affect the quality of the paper and/or the coating.
The invention also provides an apparatus for coating paper equipped with means for providing a paper web with a layer of coating composition and with a doctor extending across the width of the paper web, characterized in that means are provided for supplying in an evenly distributed manner a fluid inhibiting stalagmite formation across substantially the width of the, seen in the direction of travel of the paper web, downstream side of the doctor to an area bounded by the paper web and the above side of the doctor.
Surprisingly, it has been found that under conditions of practice stalagmite formation is prevented by the steps according to the invention without affecting the composition and/or the behavior of the coating composition in any manner whatsoever.
It is an additional advantage of the invention that the working life of a doctor blade considerably increases because this blade need no longer be replaced as is customary when too much stalagmite has been formed, but since no stalagmite is formed anymore this blade needs to be replaced only when the edge of the blade is going to exhibit signs of wear that are inadmissible for other reasons.
Different types of doctors are known and although, for simplicity""s sake, reference is always made in the present text to a doctor blade, it is emphasized that the invention is not restricted to the use of doctor blades only, but is applicable in all types of doctors extending across substantially the width of a paper web and exhibiting the problem of stalagmite formation.
It is further observed that U.S. Pat. No. 3,152,918 discloses that in the paper coating operation the properties of the coating composition are affected by supplying moisture to the area between the premetering device and the doctor blade through a pipe provided with sprayer orifices. This principle, however, is completely unsuitable when a coating composition having a high solids content is to be used with the doctor blade, while the problem of stalagmite formation is exactly greatest at such a high solids content.